The term 'staybolt' signifies strength. Stay, meaning to not move or go anywhere and bolt is well, a bolt! Used to hold things together and keep them tight.
This question will prove my knowledge of steam mechanics or as I should say, lack there of.
What exactly IS a flexible staybolt? If a staybolt (or any other bolt for that matter) implies strength and means to not go anywhere, what makes a flexible one flexible?
Are there 'non-fliexible' staybolts?

This should help shed some light on things, in a pictorial fashion:

http://www.psrm.org/faqs/boilers/page45.html

Fig. 40 is a regular staybolt

Fig. 41 is a flexible staybolt

Basically, regular staybolts are stiff, flexible staybolts allow for movement due to thermal expansion, etc. They're more like ball-and-socket joints.

Why is the flexible staybolt only "flexible" at one end?

Basically, there are two parts to a fire tube locomotive boiler from a design standpoint: 1) Unstayed areas, and 2) Stayed areas. An Unstayed area is one where the boiler plates are in their stable state from the pressure exerted upon them by the water/steam. These areas are round....the boiler barrel, the steam dome, etc. Flat areas that want to bulge due to the internal pressure must have additional parts that hold them in place to prevent bulging. Hence these parts "stay" the sheets in place. There are several types of stays in a locomotive boiler, including the shorter pieces found in the firebox area. In the days of boiler construction prior to electric welding, these "stays" were threaded rod that passed through theaded holes in both sheets holding all pieces rigid. These are regular, or rigid STAYBOLTS.

As mentioned in the prior post, the inside firebox sheets will expand more than the outside (cooler) sheet. The "VEE" area at the root of the thread of the staybolt would become sensitive to cracking as the plates move ever so slightly relative to one another in many cycles. The solution is a bolt that will "go with the flow" as these plates move. The ball-and-socket works well, but is limited to the outside of the boiler where temperatures are cool. The very hot firebox sheet must conduct the heat of the fire to the water to keep the metal from overheating......if a ball-and-socket were placed within the firebox it would simply burn off.

The inside firebox sheet is thinner than the outside sheet, and is held in place by the stays holding it to the outside sheet. Since the metal is thinner, it can flex slightly better than the thicker more rigid outside sheet. It can flex better also because it is hotter. The threads that are rigid in the firebox sheets usually won't break as quickly as the cooler more rigid end, so the rigid attachment to the thinner, hotter, firebox sheet is acceptable, and works OK.

Additionally, when the threaded bolt is screwed into the plate, it is "upset" or made fatter, within the sheet by hammering on the end. This is called "driving" the bolt, and has to be done on both ends of a rigid bolt (screwed into two sheets) or on one end of a flexible bolt (screwed into one sheet). A "dolly bar" or "buck bar" is held against the opposite end of the bolt to absorb the hammer blows from a hand or air hammer, to keep from knocking the bolt out of the sheet threads. Sometimes this "buck bar" is an air tool, which holds against another surface to lock the bolt tight.

The socket of the flexible bolt has a copper gasket and a screwed on/in cap. The cap creates a steam tight seal.

Staybolts, either rigid or flexible, should be slightly weaker than the plates they support. In other words, the metal must be softer, yet strong. The idea is to get the bolt to flex more than the plate.

Mark, Are stays designed weaker in order to absorb not only the motion of the inside sheets being hotter than the outside, but are they also designed to yield and break when there is too much movement, such as bulging, thus serving as an indicator of trouble happening or about to happen? An analogy might be shear bolts whose intended purpose is to break so as not to allow damage to other moving parts.

Also, the stay caps on SVRy No. 19 are not screwed on, but welded on.

Thanks,

dan

Staybolts are meant to be weaker to absorb movement, but not to break as any type of indicator. That said, it is still better to break a staybolt than crack a sheet. There are small holes in the ends of the staybolts that are called "telltale holes". These will allow water and/or steam to excape from the boiler if the bolt is in fact cracked or broken.....thus an indicator.

Yes, some designs of flexible staybolts have the caps welded on. They are a pain in the butt to work on.

Mark

Mark,

Thanks for the reply. My father was a Boilermaker during the steam era. I recall one morning as I woke to get ready for school, he had just arrived at home and was eating breakfast. His face was sooty and greasy. He said he had been called to work at 2 AM to replace several broken stays in an O&NW Mikado. The engineer refused to move the locomotive until it was repaired. An interesting process in that the boiler was drained while he was getting dressed, fans were used to force cool air into the firebox while by dad replaced the broken stays. I think there were about four broken. When finished, the boiler was filled again, it was force fired, and left town around 7 or 8 AM as I recall.

dan

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